# Determine volume of HCl added to propionic acid to get pH=1

1. Aug 4, 2011

### BThomas1219

1. The problem statement, all variables and given/known data
Given 250ml of 0.1M HC3H5O2 (Ka=1.35E-5) What volume of
a)1.0M HCl is needed to lower the pH to 1.00.
b) 1.00M NaC3H5O2 raise pH to 4.00

2. Relevant equations
pH=pKa-log (HA/A)
ICE chart

3. The attempt at a solution
I used the ice chart for the dissociation of propionic acid. I determined the concentration of H+ and A- to be 1.16E-3. the pH was 2.93. I do not know how to factor in the HCl to determine the volume needed to lower the pH.

2. Aug 18, 2011

### Staff: Mentor

$$K_a = \frac {[A^-][H^+]}{[HA]}$$

rearranged:

$$\frac {[A^-]}{[HA]} = 10^{pH - pK_a}$$

At pH 1.0 only about 101-4.75=10-3.75=2x10-4 of the acetic acid is dissociated. That means in the first case you can - more or less safely - assume presence of acetic acid doesn't matter much, as the first approximation pH will depend mostly on the HCl dilution.

Second case is just a buffer solution, use Henderson-Hasselbalch equation.